Sound transmission system



Oct. 14, 1941. l Y w B -sNiw 2,258,662

SOUND TRANSMISSION SYSTEM A r-TDRNE y w. B. sNo w 2,258,662

SOUND TRANSMISSIQN SYSTEM Patented Qct. 14, 1941 SOUND TRANSMISSIONSYSTEM William B. Snow, Summit, -N. J., assignor to Bell' TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication April 5, 1940, Serial No. 328,078

8 Claims.

This invention relates to systems for the reproduction of sound and moreparticularly to systems winch reproduce the sound with the spatialdistribution of the original sound. The invention applies to systems inwhich the sound is detected and transmitted for immediate reproductionand also to systems in which the sound is detected and recorded forsubsequent reproductions.

Sound transmitting systems and sound recording and reproducing systemshave been proposed heretofore in which the volume range capacity of thetransmitting medium or the recording medium is effectively increased bycompressing the volume range before transmission or re-V cording andexpanding the volume range in a complementary manner before thereproduction of the sound. Typical systems of these types are disclosedrespectively in Patents 1,565,548, December 15, 1925, A. B. Clark; and1,623,756, April 5, 1927, C. F. Sacia. In systems of this character theeffective amplification, that is, the gain or loss in the transmissionchannel, is varied 'in such a way that the louder sounds do not overloadthe system and the faintest sounds are amplified sufliciently to be wellabove the level of extraneous noise originating in the system.

It has also been proposed heretofore, to reproduce sounds in theiroriginal spatial relation by using a plurality of separate transmissionchannels with the reproducers disposed with respect to the desiredvirtual source of the reproduced sound in a manner corresponding to therelation of the microphones to the actual source of the sound. Systemsof this character are shown, for example, in British Patent 23,620 of1911, A. Rosenberg, and United States Patent 1,589,139, June 15, 1926,E. H. Foley. To produceV a satisfactory illusion of the spatialdistribution of the sound, the transmission system, which may inf cludea sound recording and reproducing.` link, must be capable of reproducinga very wide range of both the volume and the frequencies of the sound.

The object of the present invention is a multichannel stereophonic soundtransmission system capable of transmitting a very wide range of bothvolume and frequency of sound.

A feature ofthe invention is the use of volume compression and expansionindividually `in each channel of the system to increase the effectivevolume range capacity of the system.

Another feature of the invention is the use of a constant frequencycarrier current individual to each channel, varied lin accor degree ofcompression applied to sound.

A further feature of tnemventio is in use ofthe same constantfrequencyivarying ampli--v tude carrier current to control both`tha-,com-

pressor in the transmitting channel and ,the ex,-

pandenin the-reproducing channel.

A further feature of the invention is the transe.

mission of all of the'control currents fromthe dance withthe side of thedesired odd harmonic. 'Ihe upper various channels over a singletransmission channel, or the recording of 'all of the control currentsFig. 1l alagrammauc'any discloses the sending end of a transmissionsystem embodying the invention; and

Fig. 2 diagrammatically discloses the receiving' end of a transmissionsystem embodying the in' vention. Y l

'I'he microphones l, 2 are appropriately disposed in the vicinity of thesource of sound to form a stereophonic pick-up system. The sound wavesare detected by the microphones l, 2 in the form of electrical waveswhich are amplied in the ampliers 3, l, compressed in the compressors 5,6, further amplied if desired in the ampliers 1, 8 vand applied toseparate transmission channels which may, if desired, be in the form ofthe recording devices '9 and l0. Carrier currents of constant frequencyare generated by the oscillators Il, I2, modulated bythe controlcircuits I3, Il, and, after modulatiomrare transmitted over a commoncircuit which may, if desired, have the form of the common recording`device I5. The oscillator associated with any given channel generates afrequency which differs from the frequencies generated by theoscillators asso` ciated with the other channels. If desired, the vcarrierf'frequenciesmay be harmonically related, in which case theindividual oscillators may be replaced by a common oscillator and aknown form of harmonic generator with suitable filters for each channelor by an oscillation generator capable of producing a plurality ofharmonically related frequencies. Preferably the'carrier `frequenciesform a series 4of odd harmonics, because, when thus related, the beatfrequencies due to any intermodulation between the carrier frequencieswill form a series of even harmonics which may easily be eliminated bythe band-pass lters. :Ihe pass bands of the respective lters will passthe desired odd harmonic but are too narrow to pass the even harmonicslying on each thetransniitte y Other forms oi modulator, such as theonedis- 2 y 2,258,662 harmonics are also s'o phased with respect to theclosed in United States Patent 1,936,176, Novemlower harmonics that theysubtract from the ber 21, 1933, R. R. Scoville, may be used in placepeak value of the lower harmonics thus reducing of the modulator shown.

the maximum .load impressed on the recording A portion of the output ofthe modulator i3 device I5 and increasing the average load on the 5 isselected by the band-pass filter 42 and supplied recording device. tothe ampliiler-detector 43 where it is converted 'I'he records producedby the recorders 3, I0 into aslowly varying unidirectional current.'I'he and I5, may conveniently be placed side by side slowly varyingunidirectional current in the outupon the same medium I5 which may havethe put oi' they amplifier-detector 43 is supplied to `form oi' aphotographic nlm. 'I'he records on l0 the compressor 5. The compressor 5is a modithe medium I5 may be detected by the reproned form of thedevice disclosed in United States ducers I1, |3, ampliiled if desired inthe ampli- Patent 2,035,263, March 24, 1936, R. W. Cushman ers Il, 20,expanded in the expanders 2|, 22, et al., and is essentially a balancedresistance further. ampliiled if desired in the amplifiers 23, padformed by the series resistors 44. 45, 43, 41

24 and reproduced as sound waves by the repro- 15 and the shunt networkformed by the resistors f ducing devices 25, 25. The reproducers 25, 2543 and 45 in series with the non-linear resistance are related to theapparent source of sound in network 53. The slowly varyingunidirectional the same way that the microphones I, 2 are recurrent inthe output of the amplifier-detector lated to the real source of sound.All ot the 43 is supplied to the potentiometer 5| and ilows controlcurrents are reproduced by the repro- 2o through the element of thenetwork 55 back to ducer 21, amplified if desired by the amplifier 23the amplifier-detector 43. These currents ilowand supplied to suitableband-pass iilters 29, ing in the network 50 change the resistanceV ofwhich select the appropriate carrier frequency the network 50, thusvarying the shunt resistance and supply the current to control theexpanders of the resistance network and effectively varying 2|, 22. Iidesired the sound recorders and repro- 25 the loss introduced bythe-compressor 5 in the ducers may be omitted, and the channelsconchannel ofthe microphone I. 'I'he potentiometer nected directly fromthe input of the recording 5| may be adjusted to improve the balance ofdevices 9, and I5, respectively. to the output the network 50. Thetransformers and resistance of the reproducing devices I1, I 3 and 21,respecnetwork at the input and outputof the device 5 tisfely. 30 preventthe changes in the impedance of this A portion of the electrical wavesin the output device from being reilected into the channel. of theampliiler 3 are supplied to a conventional The compressor l, which isessentially a variatype of amplifier-detector 3| and are there conbleloss device, may .be replaced by a variable verted into a unidirectionalcurrent varying as gain device such as disclosed in United States somefunction of the variations in amplitude of Patent 1,623,756, April 5,1927, C. 1". Sacia. the electrical waves inthe output of the .ampli- Thecompressor I in the channel leading from iier 3. The varyingunidirectional control curthe microphone 2 may be oi the same type asrents are supplied to the modulating device 3 the compressor 5 and iscontrolled by the output in the output of the oscillator I I. The moduofthe modulator I4 which may be o! the same lating device I3 is a modifiedform oi' the device 40 character as the modulator I3. The varyingdisclosed in French patent ot addition 39,757 pubcarrier vcurrents fromthe modulators I3 and I4 lished March 12, 1932, and in United StatesPatare combined into one channel for transmission ent 2,034,703, March24, 1936, F. W. Metzger. The and may. if desired. be recorded by asingle modulator I3 comprises two series resistors 32, 33, recordingdevice I5. respectively shunted by non-linear resistance ele- While atwo-channel stereophonic system has ments 34, 35 having the property ofchanging for convenience been disclosed, it is, o! course, theirresistance with the current ilowing through evident that three or morechannels may be used, the element. The non-linear resistance elementsthe three or more control currents being combined 34,35 may convenientlycomprise a plurality ofthe and transmitted in a single channel, orrecorded known copper, copper-oxide couples. The slowly by a singlerecording device I5; varying unidirectional current from the ampli- InFig. 2 the proper control current in the outfier-detector 3| flowsthrough the resistor 35 to put of the ampliiler 23 is selected by thebandthe junction of the resistors 31 and 38 and there .pass iilter 25and supplied to an amplier-detecdivides equally. One half of the currentflows tor 52 where the control current is converted into through theresistor 31 through the resistors 33 .55 a slowly varying unidirectionalcurrent, which and 35 in parallel and through the resistor 39. ilowsfrom the amplifier-detector 52 through the The other half of the currentows through the network formed by the expander 2| back through resistor35, the resistors 32 and 34 in parallel the ampliiler-detector 52. Theexpander 2| is oi' and the resistor 40, the combined current ilowingessentially the same construction as the modulathrough the resistor 4Iback to the amplier- 6'0 tor device I3 and operates in a similar manner:detector 3|. 'Ihe unidirectional current owing thus a detaileddescription oi' the operation of in the resistors 34 and 35 changes theresistance this network iS believed t0 be unnecessary. The of theseresistors and thus changes the loss sufpotentiometer and variableresistance shown in iered by the constant frequency current from thecircuit of the device 2| assists in balancing the oscillator in passingthrough the device the Circuit The expander 2| 'ots the current I3. Thecurrent at the output of the device I3 .transmitted through thedeviceinamanner which will then have the form of a constant frequency isthe inverse oi' the eilects of the compressor l carrier current varyingin amplitude in accordon the current transmitted through that netancewith the variations in the unidirectional work. Inother words, thecompressorlinsertsa current from the amplifier-detector 3|. The 70 lossin the channel which increaseswith an intransformer and resistance padsat the input crease in theam'plitude of the current transand output ofthe device I3 prevent the variamitted through the channel. while theexpander tions in the impedance of the device I3V from 2| introduces aloss into the reproducing channel being reilected into the other part ofthe circuit. which decreaseswith an increase in theamplitudeotthecurrentiromthemicrophone w\ 3. In a transmission system,arsource of sound,

As the compressor Bris controlled by a slowly varying unidirectionalcurrent which is a function of the electrical wave from the amplifier 3,it is evident that this compressor could be controlled directly by theoutput of the amplifierdetector 3|. In the present system, however, theoutput of the amplier-detector 3| is rst converted into the form of aslowly varying carrier which is selected by the band-pass filter 42 andthen converted back into a unidirectional current by theamplifier-detector 43 and it is the output of the amplifier-detector 43which controls the compressor 5. A very real advantage is gained by thismethod of control. If the compressor 5 were to be directly controlled bythe output of the amplifier-detector 3|, and the expander 2| iscontrolled by the carrier currentfrom the modulator I3, this wouldrequire that the modulator I3 and oscillator Il operate as strictlylinear devices so that there would be exact proportionality between theoutput of the amplifier-detector 3| and the output of theampliiier-detector 52. However, in the present system, both thecompressor 5 and the expander 2| are controlled by unidirectionalcurrent derived from the same carrier output of the modulator device I3,thus it is not necessary that the oscillator vIl and modulator device |3be linear in their operation and one diflicult circuit requirement hasbeen eliminated. In either case, it is, of course, essential that theexpander 2| should be a fairly exact inverse of the compressor 5.

What is claimed is:

1. In a transmission system, a source of sound, a plurality of detectorsfor converting sound waves from said source into electrical waves, aplurality of transmission channels respectively associated with saiddetectors, means individual to each of said channels for controlling thetransmissionlof electrical waves through said channel, a plurality ofsources of electrical currents of different constant frequencies, onesource being associated with ach transmission channel, means individualtc each of said sources of constant frequency current for controllingthe output of said source in accordance with a function of the electricwave transmitted through the channel, means for combining the output ofall of said control means in a single transmission channel separate fromthe channels transmitting said electrical waves.

2. In a transmission system, a source of sound,

ulated output, and means for deriving a currentV varying with theenvelope of the amplitude variawaves from said source into electricalwaves, a

plurality of transmission channels respectively associated with saiddetectors, means individual to each channel for compressing the range ofamplitude of the electrical waves transmitted through the channel, a.plurality of sources of electrical current of constant frequencyrespectively associatedwith said channels, means for deriving Ifromy theelectrical Waves in each channel, a current varying with a function ofthe amplitude variation of the Waves in the channel, means individual toeach source of current of constant frequency for controlling the outputof said source in accordance with the current derived from the Waves inthe associated channel,

vmeans for controlling the compressor in the a microphone forvconverting sound waves from said source into electrical waves,a-reproducer for convertingsaid electrical waves into sound, and atransmission channel connecting said microphone and said reproducer andhaving a compressor associated with said microphone and an expanderassociated with said reproducer, a carrier current generator associatedwith said chan..` a

nel, means associated with the output of said microphone for deriving acurrent varying with a function of the amplitude variations of theelectric waves from said microphone, means operated by said derivedcurrent for controlling the output of said carrier current generator andmeans for controlling said compressor and said expander by thecontrolled output of 'said carrier current generator. 1

4. In a sound recording system, a plurality of spaced microphones, arecorder for each microphone, a separate circuit including control meansconnecting each microphone with the corresponding recorder, a pluralityof sources of current of different constant frequencies, means forderiving currents varying with the envelope of the currents in theseparate circuits `and respectively modulating the outputs of saidsources, and means for. recording all of said modulated outputs in asingle record track and respectively controlling the control means inone of said circuits by the modulated output of the source correspondingto said circuit.

5. In a sound recording system, a microphone, a recorder, a circuitincluding amplitude control means connecting said microphone and saidrecorder, means for deriving a current varying with the envelope of theamplitude variations of the current from said microphone, a source ofcurrent ofconstant frequency, means for modulating the output of saidsource with said envelope current, means for making a record of saidmodtions of said modulated currentand controlling said amplitude controlmeans with said current.-

6. In a sound recording system, a plurality of microphones, a recorderfor each microphone, a separate circuit including control meansconnecting each microphone with the corresponding recorder, V.aplurality of sources of currents of different constant frequencies,means for deriving currents varying with a function of the currents inthe separate circuits and respectively modulating the outputs of saidsources, and means for recording all of said modulated outputs in asingle record track.

'7. In a transmission system, a plurality of sources of electricalwaves, a plurality of transmission channels respectively associated withsaidsources, means individual to each of said channels for controllingthe transmission of electrical waves through said channel, a pluralityof sources of electrical currents of dierent constant frequencies, onesource being associated with each transmission channel, the frequenciesof said sources of constant frequency currents forming a series of oddharmonics, the upper harmonics being so phased with respect to the lowerharmonics that the peak value of the lower harmonics is reduced, meansindividual to each of said sources of constant frequency current forcontrolling the output of said source in accordance with a function ofthe electric waves transmitted through the channel, and means forcombining the output of all of said control means in a singletransmission channel separate' from the channels transmitting saidelectrical waves.

8. In a sound reproducing system, a recordingr medium comprising aplurality of records ot sound and a. separate record of a plurality oicarrier current control records, a pick-up device individual to each ofsaid sound records, a sound reproducer individual to each of said soundrecords, a transmission channel containing control means respectivelyconnecting each pick-up to the corresponding reproducer, a commonpick-up for all said control records, and means for selecting from theoutput of said common pick-up the carrier current corresponding to agiven channel and for controlling the control means in said channel withsaid current.

WILLIAM B. SNCW.

